In such a telecommunication system, the need for an equalizer arises from the different attenuation undergone by signals in different regions of the overall frequency range. That attenuation, furthermore, also varies seasonally with temperature changes affecting the transmission parameters of the line itself as well as the gain of amplifiers in repeaters along the line.
Conventional equalizers are divided into a multiplicity of stages which are individually adjustable to correct the signal level in respective frequency bands. For the adjustment of such equalizer stages it is known to send respective test frequencies over the line, these test frequencies generally lying between sub-bands used for the transmission of message signals. In long-distance telephony, for example, these sub-bands represent a multiplicity of voice channels transposed onto different carrier frequencies, with the test frequencies lying in interstitial zones separating the channels.
Situations may arise in which one or more interstitial zones are temporarily utilized for the transmission of message signals so that the test frequencies assigned to these zones cannot be sent. A case in point is the transmission of a television program over a telephone cable, the large bandwidth of the video signals requiring the simultaneous use of several voice channels for this purpose.